The present invention relates to a method of reducing the density of returning drilling mud in a riser such that it approaches the pressure of an ambient sea water column at a given depth.
The best conventional drilling practice is to use weighted drilling fluids to balance the formation pressure to prevent fracture and lost drilling fluid circulation at any depth. The weight material is often suspended bentonite or barite particles and the drilling fluid can be formulated with oil or water as a continuous phase. It should be noted that the circulation time for the complete mud system lasts for several hours, thus making it impossible to repeatedly decrease and increase mud density in response to sudden pressure variations (kicks) or lost mud circulation.
When drilling in deep water, the hydrostatic pressure of the drilling fluid column in the riser exceeds that of the corresponding sea-water column and it becomes impossible to balance the formation pressure by manipulating the mud weight. To protect the xe2x80x9copen holexe2x80x9d sections from fracture and lost mud circulation, the practice is to progressively run, and cement, casings, the next inside the previous. For each casing run, the diameter incrementally decreases until the production zone is eventually reached.
It is important that the well be completed with the largest practical diameter through the production zone. This allows high production rates to justify the high-cost of deep water development. Too small a production casing could potentially limit the productivity of the well to the extent that it becomes uneconomical to complete.
The water depth significantly affects the number of casings run and it represents one of the limiting factors in the application of conventional drilling practice in deep-water development. Various concepts have been proposed to overcome this limitation, however none of these concepts known in the prior art have gained commercial acceptance for drilling in deep waters. These concepts can be generally grouped into two categories, the mudlift drilling with marine riser concept, and the riserless drilling concept.
These concepts should not be confused with the concept of under-balanced drilling. Under-balanced drilling is basically conducted when the drilling operation is performed into the oil and gas bearing formation (pay-zone). In under-balanced drilling the hydrostatic pressure of the mud column is kept below the formation pressure in order to prevent suspended mud particles from entering and blocking the permeable oil bearing formation. Under-balanced drilling is generally prohibited and is definitely not performed outside the pay-zone for safety reason.
The riserless drilling concept contemplates removing the large diameter marine riser as a return annulus and replacing it with one or more return mud lines. Sub-sea pumps are used to lift the mud returns from the seabed to the surface. Variations over this concept are described in the following U.S. Pat. Nos. 6,263,981; 6,216,799; 4,813,495; 4,149,603. These patents generally present the same riserless system, but they are implemented using different associated pumping apparatus and/or power transmission systems. Common features are that the pump is placed at the seabed and that they all require some degree of milling or particle size reduction of the cuttings before pumping in order to avoid erosion and aggregation of the cuttings.
These xe2x80x9cpumpedxe2x80x9d systems are hampered by high cost and potential reliability problems, which are associated with the power supply and mechanics of maintaining complex sub sea systems for pumping of a suspension containing solids (cuttings).
The mudlift concept includes in principle introducing means to change the density of the returning drilling fluid at the sea bed to such a degree that the density of the fluid in the riser approaches the density of sea water.
Several mudlifit concepts are described:
1. Injecting gas (i.e. nitrogen, CO2, exhaust gas, or air) into the mud return line at various points in the riser. U.S. Pat. Nos. 6,234,258; 6,102,673; 6,035,952; 5,663,121; 5,411,105; 4,394,880; 4,253,530; 4,091,884. The concept of using nitrogen, has become common practice when an under-balanced drilling operation is performed in an oil bearing formation. These gas lift systems tend to cause sluggish pressure fluctuation when operated in deep water and are hampered by foaming and separation problems at the topside, mud separation system.
2. Diluting the mud returns with mud-base at the sea bed. U.S. Pat. No. 3,684,038. This system requires that the mud-base is recovered from the mud, which in principle means that the suspended weight material is separated from the mud. The separation process is difficult because the size of the bentonite or barite particles to be separated ranges from 1-10 micron. The concept of diluting the mud returns with mud-base to approach the density of sea water is not suitable for water-based mud since it would require infinite dilution, and for oil-based mud it would also require a high dilution ratio because of the inherently small density differences between oil and seawater. A high dilution ratio might impose dramatic changes in the rheological properties of the drilling fluid so that the carrying capacity for cuttings is lost.
3. It has been proposed to replace bentonite or barite with paramagnetic weight particles (hematite: Fe2O3) i.e. U.S. Pat. No. 5,944,195, and recover the mud-base as described above (2) with a magnetic separator. This concept has apparently not been implemented.
4. Injecting hollow glass or composite spheres in the mud return line at the sea bed. This process has been originally proposed for under-balanced drilling in U.S. Pat. No. 6,035,952, and has since also been proposed in published PCT Application No. WO 01/94740, published Dec. 13, 2001 by Maurer Technology Inc. of Houston, Tex. USA. The size of commercially available hollow spheres, manufactured by 3M Company, ranges from 10 to 100 micron with density of 0.38 to 0.53 g/cm3. These spheres are too small for efficient separation by conventional oilfield shale shakers, centrifuges or hydrocyclones, and they will collapse at pressures above 300 bar. It has however been proposed by Maurer Technology to develop and manufacture large, 10 mm, composite spheres which can tolerate pressures up to 500 bar. These spheres would have a density ranging from 0.43 to 0.68 g/cm3. They have not been put into practice and are not commercially available.
The present invention is directed to a mud-lift system based on the injection of a liquid natural gas such as NGL, ethane, propane, butane or pentane at the mud return line. The density of these liquids is in the range of 0.35 -0.58 g/cm3 under prevailing conditions, which compares favorably with hollow composite spheres since application of natural gas liquid has no upper pressure limitations. For water-based mud, the liquid is recovered from the mud in a pressurized two-phase gravity separator; for oil based mud, the liquid is recovered in a reboiler. The design of such recovery systems is basic knowledge for those skilled in the art.
The present invention offers the advantage of eliminating the need for sub-sea rotating equipment. It also offers an inherent flexibility to reach a target hydraulic pressure by selecting among different natural gas liquids (C2 through C5) and/or, by varying both the injection rate or the point of injection. The injection can also be located at multiple points in the well, which will provide means of creating a curved density gradient. It is not possible to achieve a curved density gradient in the well by the application of sub-sea pumps at the sea bed. See U.S. Pat. No. 3,684,038.